The present invention is in the field of diode encapsulation and particularly relates to glass compositions which can be used to encapsulate diodes and other small semiconductive devices at reduced sealing temperatures.
The encapsulation process for which the glasses of the invention are intended comprises the use of short lengths of glass sleeving which are placed over a diode to be encapsulated. The diode consists of a small quantity of a semiconductive material such as silicon with opposing end caps or plugs and lead wires composed, for example, of Dumet alloy. The glass sleeve and diode are then heated to a temperature sufficient to seal the tubing ends to the alloy end plugs or caps, thereby encapsulating the device.
Important properties of glasses to be utilized for this application are glass softening point, chemical durability, coefficient of thermal expansion, and chemical compatibility with semiconductive materials. The softening point should be relatively low to provide effective sealing at temperatures low enough to avoid damage to the semiconductor. The coefficient of thermal expansion of the glass should closely match that of Dumet or other relatively high expansion alloys (about 90 .times. 10.sup.-.sup.7 /.degree. C.), which are utilized for end caps and lead wires. The chemical durability of the glass should be sufficiently high to resist attack by moisture in use. And the glass should have as low an alkali content as possible, consistent with other requirements, in order to avoid contamination of the semiconductive element. However, potassium is very much less deleterious to device performance than are sodium and lithium.
When it is desired to encapsulate semiconductive devices such as diodes by the sleeving method, additional requirements must be met by the glass. The successful manufacture of the glass tubing fom which the sleeves are cut requires that the glass exhibit reasonable forming characteristics. Glasses with an excessively steep viscosity-temperature relationship (corresponding to a sharp rather than a gradual rise in viscosity as the glass is cooled) cannot readily be formed into glass tubing by commercially practicable methods. High viscosity at the liquidus temperature of the glass is also very important for tube drawing processes. Finally, good resistance to alkaline solutions is required since the tubing manufacturing process includes a washing operation in a strong detergent solution.
Lead silicate glasses have long been utilized in electrical applications because of the relatively low softening points and good electrical insulating properties exhibited thereby. U.S. Pat. Nos. 2,018,817 and 2,018,816 to Taylor, for example, disclose high expansion lead alkali silicate glasses, having softening points in the range of about 580.degree.-645.degree. C. and electrical resistivities above 10.sup.8 ohm-centimeters, which glasses are useful for fabricating electrical lamp parts.
U.S. Pat. No. 2,642,633 to Dalton describes somewhat softer (lower softening point) glasses in the PbO-Al.sub.2 O.sub.3 -- B.sub.2 O.sub.3 -- SiO.sub.2 composition field, referred to as "solder" glasses, which are typically utilized in the form of glass frits to seal together glass parts composed of soda-lime-silica or other relatively high expansion glasses. These solder glasses have the advantage of being free from alkali metals and are high in electrical resistivity, but they typically exhibit somewhat lower chemical durability (resistance to weathering), slightly lower thermal expansion, and less desirable working characteristics than alkali lead silicate glasses.
Hence, while quite suitable for the manufacture of fritted glass or pressed glass parts, the steep viscosity-temperature dependence of the aforementioned solder glasses makes them unsuitable for the manufacture of drawn glass tubing of kind useful for diode encapsulation sleeves. For these glases, an increase from softening point viscosity (10.sup.7.5 poises) to annealing point viscosity (10.sup.14.5 poises) over a cooling interval of 100.degree. C. is not uncommon. This behavior attests to the short working range of these glasses. Moreover, these glasses typically exhibit rather low viscosity at the liquidus, making tube forming difficult. Because of these characteristics, solder glasses used for diode encapsulation have generally been provided in frit form, with sleeving, if required, being produced by sintering fritted glass. U.S. Pat. No. 3,723,835 to Davis et al. illustrates the use of alkali-free lead borosilicate glass frits for diode sealing, while U.S. Pat. No. 3,535,133 to Akhtar shows the direct sealing of low expansion frits to silicon metal.
The advantages of utilizing mass-produced glass tubing to provide sleeves for encapsulating semiconductive devices are well known. Avoiding the need to manufacture, transport and handle powdered glasses, and the liquid vehicles and binders necessary to provide a sintered glass coating from such powders, are of particular commercial importance. Thus alkali-free encapsulation glasses capable of being drawn into tubing have been developed. U.S. Pat. No. 3,493,405 to Thomas describes lead aluminosilicate glasses suitable for semiconductor encapsulation, but these glasses are designed to seal to low expansion metallic components such as Kovar alloy or molybdenum. Hence only relatively low expansion glasses (45 .times. 10.sup.-.sup.7 to 55 .times. 10.sup.-.sup.7 /.degree. C.) having rather high softening temperatures (637.degree.-683.degree. C.) are provided.
For the sleeve encapsulation of semiconductive devices utilizing higher expansion metallic components such as Dumet alloys or the like, lead alkali silicate glasses exhibiting both acceptable working characteristics and reasonable softening properties are required. Glasses presently in use for this purpose are similar in properties to more complex glasses, such as described in U.S. Pat. No. 3,047,410 to Woodall et al., which were designed for use in the manufacture of electronic tubes. One glass which has been commercially employed in the manufacture of tubing for diode encapsulation consists of about 59% PbO, 35.3% SiO.sub.2 , and 5.7% K.sub.2 O. This glass has a coefficient of thermal expansion of about 91 .times. 10.sup.-.sup.7 .degree. C., a softening point of about 580.degree. C., and an annealing point of about 390.degree. C. The working characteristics of the glass are good, as is suggested by the substantial temperature interval (190.degree. C.) between the softening point and the strain point of the glass. However, still softer glasses exhibiting acceptable working characteristics as well as the required thermal expansion and chemical durability are desired.